1. Field of the Invention
The present invention relates to a compensation circuit and method, and in particular certain embodiments of the present invention relate to an overdriving circuit and method for source drivers to overdrive thin film transistors (TFTs) in a liquid crystal display (LCD) module.
2. Description of the Prior Art
As a result of smaller volume and less power consumption, flat-panel displays, such as liquid crystal displays (LCDs), have been gradually replacing cathode ray tube (CRT) displays and becoming the mainstream in the field of display devices, for example, LCD monitors, LCD televisions, and so forth. However, because of the characteristics of the molecules of the liquid crystal material, a motion blur phenomenon occurs when LCD devices are used to display high speed dynamic images or videos.
There are two common methods familiar to those skilled in the art to eliminate the motion blur phenomenon. The first method is referred to as “capacitance coupling” and the second method is referred to as “overdriving”. According to the capacitance coupling method, a circuit can be altered to meet the requirement mentioned above, but it is unable to compensate for a rising signal and a falling signal at the same time. Further, the circuit under the capacitance coupling method can not compensate for each pixel. On the other hand, the overdrive method requires more complex circuitry than the capacitance coupling circuitry. The overdrive method requires frame buffers and does not have the limitations of the capacitance coupling method. The overdrive method can use unmodified control circuits and driving circuits.
Referring to FIG. 1, a look up table (hereinafter “LUT”) typically used by source drivers to overdrive a LCD module is illustrated. In this example, a given source driver has 8-bit data so it can drive 28=256 (0-255) gray scale levels. The LUT shows overdrive gray scale values based on the current gray scale value of a pixel and the desired (next) gray scale value of that pixel. The first row (i.e., the horizontal axis) represents the starting gray scale value of a pixel before a change of data from a source driver, whereas the left-hand column (i.e., the vertical axis) represents the ending gray scale value of the pixel after the change of data. For example, according to the LUT shown in FIG. 1, when it is desired to change a pixel from gray scale level 0 to gray scale level 128, the source driver should overdrive the gray scale of the pixel to gray scale level 201 rather than gray scale level 128. For another example, when it is desired to change a pixel from gray scale level 255 to gray scale level 128, the source driver should overdrive the gray scale level of the pixel to gray scale level 61 rather than gray scale level 128.
However, it will be apparent to those skilled in the art that overdriving is limited for some “from-to” value pairs shown in the LUT of FIG. 1. For example, when it is desired to change a pixel from gray scale level 0 to gray scale level 255, the source driver would preferably overdrive the gray scale level of the pixel to a gray scale level higher than 255. The source driver in the present example, however, can only provide 256 (0-255) gray scale levels because it is limited to 8-bit data. That is, gray scale level 255 is the maximum gray scale level the 8-bit source driver can provide. The same is true for other “from-to” value pairs, such as from gray scale level 16 to 255, from gray scale level 32 to 255, etc. Similarly, when it is desired to change a pixel from gray scale level 255 to gray scale level 0, the source driver would preferably overdrive the gray scale level of the pixel to a value less than gray scale level 0. In this example, however, gray scale level 0 is the minimum gray scale level possible. A similar problem exists where a pixel's gray scale level is changed to lower values, such as from gray scale level 240 to 0, from gray scale level 224 to 0, and the like. Accordingly, the overdriving of a pixel will deteriorate when changing to higher values, such as more than gray scale level 240, and/or when changing to lower values, such as less than gray scale level 16.
One of the methods for solving the foregoing problem is to employ 9-bit source drivers, so that the gray scale of a pixel not only can be overdriven to values higher than 255 for desired gray scale values from 240 to 255, but also can be overdriven to the values lower than 0 for desired gray scale values from 16 to 0. This is because the 9-bit source driver can provide 29=512 values for the use of 256 gray scales. The extra bit can be used to transmit the compensation data for those gray scale values smaller than 16 and/or higher than 240. However, each 9-bit source driver requires a 9-bit digital-to-analog converter (DAC) which complicates the design of the circuit, makes the die size larger, increases operating voltage, and increases the cost of the chip.